msm: kgsl: Avoid NULL pointer in adreno setstate functions

Make sure the specified context_id returns a valid context
in adreno_gpummu_setstate and adreno_iommu_setstate and
silently return if it doesn't.

CRs-Fixed: 439367
Change-Id: Ic0dedbad1a91b53f19782b50380bd36842614351
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>